to PID or not to PID that is the question

rscott9399

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Hypothetical

You have an oil pump running.
You are reading a flow in GPM that is properly spanned from the device on a 4-20.

You need to add an additive to the flow. There is no flow meter on that additive so all you can do is turn the additive pump speed up and down with a 4-20 output. An operator will simply adjust on a 0 - 100% scale how fast the additive pump runs in relation to the flow.

The pump speed must be proportional to the flow.

I can do this easily with a bunch of compute statements and it turns into a math exercise.

Question : is simply doing the math the best option or is there an easy way to do it with a PID block in one rung?

any thoughts?

Thanks
 
You can't have a PID without a process variable. What is the feedback that the PID would be adjusting to?

If you just need the pump speed to be proportional to the flow, a calculation is the only way to do that.
 
You can't have a PID without a process variable. What is the feedback that the PID would be adjusting to?

If you just need the pump speed to be proportional to the flow, a calculation is the only way to do that.

As i said above, i have a process variable.
I can use the oil flow as feedback.

So obviously not typical but im curious if there is a simple way instead of the math statements
 
You do not have a feedback. You are looking the concentration of an additive but have no way of measuring that additive, therefore it is open loop and math is the way to go.
 
You do not have a feedback. You are looking the concentration of an additive but have no way of measuring that additive, therefore it is open loop and math is the way to go.

i do have A A A A feedback lol just not feedback of the pump i want to control

alright, will scrap that idea
Math it is

thanks for the ideas
 
No you do not have a feedback, you have a setpoint. The setpoint is X*oil_flow_rate. Where X is whatever ratio you have to add the additive at. A feedback would be actual additive flow rate. Is the additive pump a positive displacement pump?
 

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